Coalescence of viscoelastic sessile drops: the small and large contact angle limits

TL;DR

This paper investigates how viscoelasticity influences the coalescence of sessile drops, revealing that contact angle determines whether viscoelastic effects significantly alter the coalescence dynamics, with different behaviors at small and large angles.

Contribution

It provides a detailed analysis of the impact of contact angle on viscoelastic drop coalescence, combining experiments and simulations to elucidate the roles of Deborah and elastocapillary numbers.

Findings

At small contact angles, viscoelasticity has minimal effect on coalescence.

The Deborah number scales as θ^3 for small contact angles, explaining near-Newtonian behavior.

At large contact angles, viscoelasticity significantly alters interface dynamics, depending on Deborah and elastocapillary numbers.

Abstract

The coalescence and breakup of drops are classic examples of flows that feature singularities. The behavior of viscoelastic fluids near these singularities is particularly intriguing - not only because of their added complexity, but also due to the unexpected responses they often exhibit. In particular, experiments have shown that the coalescence of viscoelastic sessile drops can differ significantly from their Newtonian counterparts, sometimes resulting in a sharply defined interface. However, the mechanisms driving these differences in dynamics, as well as the potential influence of the contact angle are not fully known. Here, we study two different flow regimes effectively induced by varying the contact angle and demonstrate how that leads to markedly different coalescence behaviors. We show that the coalescence dynamics is effectively unaltered by viscoelasticity at small contact angles. The Deborah number, which is the ratio of the relaxation time of the polymer to the timescale of the background flow, scales as $\theta^3$ for $\theta \ll 1$, thus rationalizing the near-Newtonian response. On the other hand, it has been shown previously that viscoelasticity dramatically alters the shape of the interface during coalescence at large contact angles. We study this large contact angle limit using experiments and 2D numerical simulations of the equation of motion. We show that the departure of the coalescence dynamics from the Newtonian case is a function of the Deborah number and the elastocapillary number, which is the ratio between the shear modulus of the polymer solution and the characteristic stress in the fluid.